The present invention relates to the field of photography and, more particularly, to a cassette for releasably holding a photosensitive film unit and locating the film unit at a film plane within the cassette in preparation for exposure.
The cassette embodying the present invention is particularly well suited for use with a large format (e.g. 8.times.10 or larger) film unit that forms a separate "negative" component of a self-developing film assemblage which also includes a "positive" component comprising an image receiving sheet having a rupturable container of fluid processing composition at one end thereof.
In the illustrated embodiment, the cassette is designed for radiographic applications and includes an interior bearing surface against which one side of the film unit is to bear in intimate contact to locate it at the film plane, and an intensifying screen that is adapted to be urged into intimate contact with the opposite photosensitive side of the film unit to enhance image formation when the intensifying screen emits visible light in response to impingement of X-ray radiation thereupon. Provisions are also made for daylight loading of an X-ray sensitive film unit that is initially provided with an opaque envelope covering the photosensitive portion thereof for protecting the film unit from ambient light until it is safely located within the light excluding environment of the cassette housing.
Cassettes of this type generally include some type of mechanism for selectively applying pressure to the film unit in a direction transverse to the film plane. Initially, after the film unit is loaded into the cassette it is desirable that this pressure be at a minimum so that the opaque envelope may be withdrawn through a light sealed passageway at one longitudinal end of the cassette by advancing it in a direction parallel to the film plane. However, before the cassette is mounted on an appropriate apparatus of film exposure, it is desirable that increased pressure be applied to the film unit so that one side thereof is urged into intimate contact with the film bearing surface while the intensifying screen is urged into intimate contact with the opposite side of the film unit. Following exposure, it is desirable to once again minimize this pressure to facilitate the advancement of the exposed film unit in a direction parallel to the film plane through a light seal passageway at the opposite end of the cassette so that it may be advanced into superposition with the image receiving sheet for processing whereby the superposed sheets are advanced between a pair of pressure applying rollers in a film processor for releasing the fluid from the rupturable container and distributing it between the superposed negative and positive sheets to initiate a well-known development and diffusion transfer process.
Cassettes designed for use with large format self-developing film units are, of course, well-known in the prior art.
One type of cassette includes a base section and a cover section that are coupled together for movement between an open position for film loading and a single closed position wherein structure associated with the cover section applies a given pressure, in a direction transverse to the film plane, for urging one side of the film unit into intimate contact with a film plane defining bearing surface and for urging the intensifying screen into intimate contact with the opposite side of the film unit. For representative examples of this type of cassette, see U.S. Pat. Nos. 2,689,306; 3,371,208; 3,383,507; and 3,804,625. However, these patents disclose that the amount of pressure applied is critical and must be balanced to provide enough pressure for good intimate contact and yet not so much as to inhibit the withdrawal of the opaque envelope or the subsequent advancement of the exposed film unit from the cassette for processing. While this type of structure may work well, it will be obvious to those skilled in the art that the individual piece parts of the cassette must be manufactured to relatively high tolerances to maintain the critical pressure thereby increasing the manufacturing cost of these cassettes. Also, one may expect that the normal wear and tear on the cassette inherent in extended use may adversely affect the cassette's ability to maintain a pressure in the critical range.
Another type of large format cassette includes a base section and cover section that are coupled together for movement between an open position for film loading and a closed position wherein the cassette is rendered light tight and a minimum of pressure is applied to the film unit in a direction transverse to the film plane to facilitate envelope withdrawal and the subsequent removal of the exposed film unit for processing. Additionally, these cassettes include a movable pressure mechanism and some type of actuating system which allows the pressure to be suitably increased for good intimate contact of the film unit with the bearing surface and the intensifying screen with the photosensitive side of the film unit. For example, see U.S. Pat. Nos. 2,709,223 and 3,103,864 wherein the cover section of the cassette includes a rectangular frame having a movable pressure plate mounted therein and a cam and lever arrangement that is actuable to load the pressure plate and urge it toward the film plane to provide the required pressure. Also see U.S. Pat. No. 2,823,317 where the opening in the frame of the cover section is covered with a flexible diaphragm which is deflected by a platen and clamp mechanism to increase pressure in the direction transverse to the film plane. U.S. Pat. No. 3,174,039 discloses a cassette having a lever mechanism which is actuated to move a pressure plate associated with the cover section. Still another type of cassette, disclosed in U.S. Pat. Nos. 3,327,155 and 3,330,953, utilizes an inflatable air bag for increasing the pressure in a direction transverse to the film plane. Also see commonly-assigned copending application U.S. Ser. No. 841,889 now U.S. Pat. No. 4,186,308 filed on Oct. 13, 1977 by Herman E. Erikson which discloses a cassette having a movable pressure plate disposed between fixed exterior cassette wall and an actuating cam system for moving the pressure plate toward and away from the base plate.
While these cassettes do provide the desirable feature of being able to vary the pressure for good intimate contact on one hand and withdrawal of the envelope and film unit on the other hand, the addition of the pressure plate or air bag and the required actuating mechanisms for these devices generally make these cassettes more mechanically complex and expensive to manufacture than is desirable.
Therefore, it is an object of the present invention to provide a cassette that is suitable for use with a large format film unit, allows pressure thereon in a direction transverse to the film plane to be varied, and yet is of simple construction and low cost.
It is yet another object of the invention to provide such a cassette that doesn't require the use of a movable pressure plate per se and its accompanying actuating mechanisms.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.